1. Field of the Invention
The invention relates to a pregripper configuration for single-sheet feeding to a rotating cylinder of a sheet-fed rotary printing press, generally with a pregripper pivot arm pivotally mounted at a printing press frame, a cam drive with a cam, a roller lever which is pivotable about a pivot shaft which is fixed relative to the printing press frame, and lever means articulatingly connected between the roller lever and the pregripper pivot arm for transmitting a motion of the roller lever to the pregripper pivot arm.
2. Description of the Related Art
A pregripper drive for such a pregripper is known from U.S. Pat. No. 4,993,275 to Pollich et al. of Heidelberger Druckmaschinen. In addition to a main cam disk for driving the pregripper, a complementary auxiliary cam disk is disposed on the same shaft, with the result that two rollers, held at the ends of a angular double-crank roller lever, positively guide the roller lever motion. According to that patent, the roller lever, in turn, is disposed in a fixed manner on an oscillatingly moved pregripper shaft or its motion is transmitted by a lever mechanism to a rocker arm. That rocker arm is held at one end on a frame-fixed shaft and it carries the pregripper mechanism at its other end.
The transfer of a sheet from a feed table to a cylinder of the printing press (the cylinder, of course, rotates at printing-press speed), places particular demands on the pregripper drive and on other main and auxiliary components. Firstly, the previously stationary sheet must be accelerated to the peripheral speed of the cylinder. Secondly, the sheet must be positioned extremely accurately with its leading edge, so as to make a secure transfer to the gripper mechanism of the cylinder possible. That accurate sheet positioning must also be guaranteed in the case of the repeated passage of the sheet at different press speeds.
Undesirable dynamic effects, particularly vibrations, extreme peak values of the periodic driving torque at the pregripper drive and similar effects, must be prevented by providing small acceleration peaks in the swivel motion of the pregripper. This can be achieved for the aforementioned operation, for instance, by providing a large printing press angular interval and by a long transfer path from the feed table to the cylinder. A desirable stable construction of the pregripper is assisted by choosing a short pregripper, i.e. by providing a short distance between the gripper clamp and the pregripper pivot point. This, however, means that, in order to produce the periodic motion of the pregripper, it is necessary to have a non-uniformly acting drive mechanism with a plurality of members. All of the members are interconnected by joints, which, even in the case of positive form-locking construction, are subject to play.
Play in the joints--due to the periodically changing forces in the joints--causes backlash or contact changes in the joint elements. This leads to changes in the transmission behavior of the pregripper drive. Register inaccuracies are the result. A periodically changing driving torque causes irregular printing-press operation, which is detrimental to the quality of the printed products.
In order to eliminate play in the joints of articulated drives, it has been heretofore known, for example, to employ torsion springs, through which the parts of the lever mechanism are braced together. Such a system is shown in German published, non-prosecuted application DE 39 22 186 A1 (corresponding the UK patent application 2 233 728 A). It has also been known to provide a longitudinal spring instead of such torsion springs. This spring, however, is so disposed that torques occur at the drive shaft of the pregripper drive and of a compensating drive. These torques cause apparent power and power losses in the printing press. In the case of a symmetrical pregripper motion, for instance, roller forces at the pregripper drive are necessarily doubled. For the purpose of power compensation, it is known from the aforementioned DE 39 22 186 A1 to provide a compensating drive, which acts directly on the pregripper pivot arm. This minimizes the forces in the pregripper drive. The motion characteristic during the forward and backward movement of the pregripper, however, is identical, with the result that, in order to obtain the maximum effect, the law of motion of the pregripper must be symmetrical. Other devices for power compensation involving a very high structural outlay are known from German published, non-prosecuted applications DE 41 09 409 A1 and DE 41 09 824 A1. Configurations for power compensation according to German patent DE 12 63 434 B2 and according to U.S. Pat. No. 4,854,236 to Thinker et al. are external. This has the effect that a combination with means for the compensation of play in the drive is not possible.